lib/test: rename dotted-self-import tests to <mod>_test (#16 PREP-a2)

package L + import P.L is self-import in Go terms regardless of path
spelling (the parser drops the dotted prefix); the text-level
import-string!=package-name comparison hid these 3 from the PREP-a set.
All three verified black-box (qualifier-only exported-API use) — plain
rename, dotted imports kept as legit cross-package.

lib/math/checked/checked_test.ww

@@ -1,4 +1,4 @@
-package checked;
+package checked_test;
 
 // Directory import pulls both checked.ww and saturating.ww (the two
 // files of this module); a bare `import checked` would resolve only to